The present invention relates to a method and system for dehumidifying a gas stream consisting of a mixture of air, metabolic carbon dioxide and water vapor; removing the carbon dioxide from the dehumidified gas stream; and then re-humidifying the CO2-free air stream. More particularly, this invention relates to a method and system for reconditioning the atmosphere in a closed habitable environment in a relatively passive manner.
The breathing air of a closed inhabited spacecraft quickly reaches unacceptable levels of humidity and carbon dioxide unless controlled. Current technology utilizes combinations of regenerable adsorbent beds to remove both CO2 and water. Typically, two or more beds are operated in a cyclic mode wherein one bed is adsorbing while the other bed is desorbing. Desorption is achieved by heating and/or pulling a vacuum on the bed. Both molecular sieves and solid amine adsorbents will adsorb both CO2 and water. If the carbon dioxide is to be dumped to a vacuum, then the water adsorbed with it will be lost. The atmosphere in the space craft then tends to get overly dehydrated to the point that it is uncomfortable for the crew members. Furthermore, as manned space missions become longer, the need to recover water and achieve closed loop water balance becomes more critical. Thus, one problem relating to space missions deals with the need to humidify the atmosphere in the space craft.
There are several methods of recovering the water from the closed habitable craft atmosphere before the water is lost. One method is to adsorb the water first on a desiccant prior to its introduction to the CO2 adsorbent. Desiccants, such as molecular sieves or silica gel, require high energy for desorption. Another method is to condense the water. This requires heat removal and places a high load on a cooling system to affect the phase change. Additionally, in a weightless environment, the air stream with the condensed water must be processed by a separator adding complexity to the entire system.
It would be desirable to provide a simplified essentially passive dehumidifying and re-humidifying system and method for treating the breathable atmosphere in a closed habitat of the type described above.
Our solution to the problem of dosed habitat breathable atmosphere humidity control is to remove and then replace water vapor in a habitat atmosphere exhaust stream bled from the habitat. Water vapor removal is accomplished by means of a membrane which will selectively extract water vapor from the habitat exhaust gas stream so as to convert the habitat exhaust gas stream to an essentially dry gas stream which contains oxygen and CO2. The water vapor removal step is performed prior to the habitat exhaust stream""s entering the CO2 adsorbent bed. Any remaining water vapor in the gas stream is absorbed by the adsorbent bed. The water vapor passes through the membrane. The membrane is contained in a membrane module or housing and the membrane divides the housing into two chambers, wherein the chamber that receives the exhaust from the adsorbent bed has a lower water vapor partial pressure than the other chamber.
The air-water vapor-CO2 mixture is drawn from the habitat and passes into one of the chambers in the membrane housing. As the mixture flows through the first chamber, water vapor is removed from the mixture by the membrane and the water vapor passes through the membrane into the second chamber. As the air stream moves through the first chamber in the membrane housing, the water vapor content of the stream decreases to a minimal amount. The resultant mixture of air and CO2 and a small amount of water vapor in the first chamber then flows to a CO2 adsorption station where the CO2 and the remaining water vapor are removed from the mixture. The adsorption station can take the form of a CO2 adsorption bed or some other form of conventional CO2 scrubber material. After the CO2 and the remaining water vapor are removed from the gas stream, the resultant air stream is drawn through the second chamber where it will become rehydrated by the water vapor that has passed through the membrane and into the second chamber. As the returning air stream flows through the second chamber, counter current to the direction of flow through the first chamber, the water vapor content of the stream increases. The return air stream always contains less water than the inlet air stream, thus providing the driving force for water vapor to permeate through the membrane. The rehydrated air stream is then returned to the habitat for breathing purposes.
The system of this invention is relatively passive in that significant energy input is not necessary to enable operation of the system. The small amount of water vapor removed in the CO2 adsorbent bed is needed to create the driving force for the membrane to operate as intended. The larger the area of the membrane, the smaller the amount of water vapor absorbed by the CO2 adsorbent bed must be. For complete water vapor recovery, it is possible to include a small regenerable desiccant bed prior to the CO2 adsorbent bed.
It is therefore an object of this invention to provide a method and system for conserving the water vapor content, while removing the CO2 content, of an environmental gas mixture stream which is extracted from a closed habitable environment, such as a space craft, space suit, submarine, space station, or the like.
It is an additional object of this invention to provide a method and system of the character described wherein the water vapor content of the breathable atmosphere is removed from an air-CO2-water vapor gas mixture that is extracted from the habitable environment.
It is a further object of this invention to provide a method and system of the character described wherein the water vapor content is removed from the gas mixture by means of a membrane which selectively passes water vapor in the gas mixture from one side of the membrane to an opposite side of the membrane.
It is another object of this invention to provide a method and system of the character described wherein the dehydrated air-CO2 mixture has the CO2 and a small amount of water vapor content thereof stripped from the air content, and the CO2-free air is then rehydrated by water vapor that was removed by the air-CO2-water vapor gas mixture by the membrane.